The production of organic compounds using synthesis gas, which is a mixture of carbon monoxide and hydrogen, or from carbon monoxide as one of the reactants has been known for a significant period of time. It is well known that one can produce methanol directly from synthesis gas and that methanol can be further reacted by hydroformylation, homologation and carbonylation reactions to produce acetaldehyde, ethanol and acetic acid or its methyl ester, respectively. It is also known that alcohols, esters, ethers, and other organic compounds can be reacted with synthesis gas or carbon monoxide to produce oxygenated organic compounds. The difficulties, however, have resided in the ability to carry out any one of these chosen reactions to produce the desired compound at acceptable efficiency, conversion rate and selectivity.
In almost all instances the reaction is generally catalyzed using a Group VIII transition metal compound as the catalyst and a halogen as the promoter. It is known that many other metal compounds and promoters can be used. In addition, the prior art has disclosed the use of secondary activators or ligands in conjunction with the metal catalysts and promoters. These secondary activators can be other metallic salts or compounds, amines, phosphorus compounds, as well as a multitude of other compounds that have been disclosed in the published literature. Thus, a typical catalyst system contains the metal atom catalyst, promoter and, optionally, ligands, solvents and secondary activators. Though a significant amount of literature does exist describing the production of acetic acid, to our knowledge it does not disclose or suggest our invention. Several of the pertinent patents in this area are discussed below.
French patent No. 2,317,269, filed by Compagnie Des Metaux Precieux and published on Feb. 4, 1977, discloses the production of aliphatic carboxylic acids by the reaction of an alcohol with carbon monoxide in the presence of a catalyst containing at least three essential components, iridium atom, copper atom and halogen. This is not our process.
In European patent application No. 0,018,927; filed by Gauthier-Lafaye et al on Apr. 23, 1980 and published on Nov. 12, 1980, there is described a process for the production of monocarboxylic acids by the carbonylation of an alcohol using a nickel catalyst, a halide and a solvent. In the instant process an organic acid is produced from an alcohol and carbon monoxide in contact with an organic ester source using a rhodium complex as the catalyst and a specific halogen source, such as lithium iodide and methyl iodide.
U.S. Pat. No. 3,060,233, issued to Hohenschutz on Oct. 23, 1962, discloses the carbonylation of methanol to acetic acid using a metal of the iron group of the Periodic Table and a halide. It does not disclose use of the instant rhodium complex at the mild pressure and temperature conditions employed herein.
U.S. Pat. No. 3,769,329, issued Oct. 30, 1973 to Paulik et al, discloses the use of a conventional rhodium catalyst and conventional ligands. The preferred mode of operation of this prior art process requires a large excess of water to ensure selectivity to acetic acid. This reference employs relatively extreme reaction conditions of temperature and pressure, and makes no distinction relating to the suitability of useful ligands or halogen sources.
U.S. Pat. No. 4,212,989, issued to Isshiki et al., on Jul. 15, 1980, describes a process for producing carboxylic acids or their esters by reacting an alcohol or an ether with carbon monoxide using a Group VIII metal catalyst and an iodine promoter. The reference contains no disclosure or suggestion of the production of organic carboxylic acids employing a specific rhodium complex under mild reaction conditions.
British patent specification No. 1,584,740, issued Feb. 18, 1981 to Air Products relates to the production of acetic acid at more extreme reaction conditions using a different ligand, shown by us to be ineffective under the mild reaction conditions of this invention.
Another known procedure for producing acetic acid is the catalytic isomerization of methyl formate as shown by the reaction: EQU CH.sub.3 O(O)CH.fwdarw.CH.sub.3 COOH
This procedure is shown in U.S. Pat. No. 1,697,109, issued to Henry Dreyfuss on Jan. 1, 1929. The process described is a vapor phase isomerization reaction caried out at 200.degree. C. to 450.degree. C. at a pressure, for example, on the order of 200 atmospheres using a metal oxide or acetate catalyst. That is typical of the extreme reaction conditions normally used in this area of technology. The reference does not disclose the use of alcohols as starting materials.
U.S. Pat. No. 4,194,056, filed by Antoniades and issued Mar. 18, 1980, discloses the production of carboxylic acid from methyl formate using a soluble rhodium catalyst, halogen promoter and carbon monoxide. This is not the process of the instant invention, nor does this reference disclose or suggest the use of a specific rhodium complex, the instant mild reaction conditions or the specific halogen source employed herein nor the unexpected results achieved by their use.
It can be seen that the prior art contains many disclosures dealing with the catalytic production of acetic acid, including its production from methanol. The art also discloses the production of other organic carboxylic acids from other alcohols. One of the disadvantages of many of these reference processes is the presence of water with the eventual need to remove it from the desired organic acid product. This removal is both complicated and costly. The process of this invention generates anhydrous carboxylic acids and essentially eliminates this problem.
Frequently, as shown above, typical prior art processes employing rhodium catalyst to produce acetic acid, require rather harsh reaction conditions of temperature and pressure to obtain satisfactory yields of products. Such reaction conditions require use of expensive reactors, engender excessive energy cost, often lead to undesired by-products and cause excessive corrosion problems.
Many processes employed for the production of organic acids use a catalyst system containing a source of metal atom and a source of halide atom. The alkali metal halides are often mentioned as suitable halide sources, but no distinction is made between any specific one of the alkali metal halides or between any other halogen compound. Nor do any of the references suggest or recognize the unexpected advantage of the use of mixtures of lithium iodide and methyl iodide in conjunction with rhodium catalyst.